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1-1-2013

The densest

J. Strader Michigan State University

A. C. Seth University of Utah

J. P. Brodie Observatories

D. A. Forbes Swinburne University

G. Fabbiano Harvard-Smithsonian Center for

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Recommended Citation J. Strader, A. C. Seth, J. P. Brodie, D. A. Forbes, G. Fabbiano, Aaron J. Romanowsky, C. Conroy, N. Caldwell, V. Pota, C. Usher, and J. A. Arnold. "The densest galaxy" Astrophysical Journal Letters (2013). https://doi.org/10.1088/2041-8205/775/1/L6

This Article is brought to you for free and open access by the Physics and Astronomy at SJSU ScholarWorks. It has been accepted for inclusion in Faculty Publications by an authorized administrator of SJSU ScholarWorks. For more information, please contact [email protected]. Authors J. Strader, A. C. Seth, J. P. Brodie, D. A. Forbes, G. Fabbiano, Aaron J. Romanowsky, C. Conroy, N. Caldwell, V. Pota, C. Usher, and J. A. Arnold

This article is available at SJSU ScholarWorks: https://scholarworks.sjsu.edu/physics_astron_pub/21 The Astrophysical Journal Letters, 775:L6 (6pp), 2013 September 20 doi:10.1088/2041-8205/775/1/L6 ©C 2013. The American Astronomical Society. All rights reserved. Printed in the U.S.A.

THE DENSEST GALAXY

Jay Strader1, Anil C. Seth2, Duncan A. Forbes3, Giuseppina Fabbiano4, Aaron J. Romanowsky5,6, Jean P. Brodie6 , Charlie Conroy7, Nelson Caldwell4, Vincenzo Pota3, Christopher Usher3, and Jacob A. Arnold6 1 Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824, USA; [email protected] 2 University of Utah, Salt Lake City, UT 84112, USA 3 Centre for Astrophysics & Supercomputing, Swinburne University, Hawthorn, VIC 3122, Australia 4 Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA 5 Department of Physics and Astronomy, San Jose´ State University, San Jose, CA 95192, USA 6 University of California Observatories/Lick Observatory, Santa Cruz, CA 95064, USA 7 Department of Astronomy & Astrophysics, University of California, Santa Cruz, CA 95064, USA Received 2013 July 26; accepted 2013 August 14; published 2013 August 30

ABSTRACT We report the discovery of a remarkable ultra-compact around the massive 8 NGC 4649 (M60), which we call M60-UCD1. With a dynamical mass of 2.0 × 10 M0 but a half-light radius of only ∼24 pc, M60-UCD1 is more massive than any ultra-compact dwarfs of comparable size, and is arguably the densest galaxy known in the local . It has a two-component structure well fit by a sum of S ersic´ functions, with an elliptical, compact (rh = 14 pc; n ∼ 3.3) inner component and a round, exponential, extended 38 −1 (rh = 49 pc) outer component. Chandra data reveal a variable central X-ray source with LX ∼ 10 erg s that could be an associated with a massive hole or a low-mass X-ray binary. Analysis of optical spectroscopy shows the object to be old (210 Gyr) and of solar , with elevated [Mg/Fe] and strongly enhanced [N/Fe] that indicates light-element self-enrichment; such self-enrichment may be generically present in dense stellar . The (σ ∼ 70 km s−1) and resulting dynamical mass-to-light ratio (M/LV = 4.9 ± 0.7) are consistent with—but slightly higher than—expectations for an old, metal-rich with a Kroupa initial mass function. The presence of a massive or a mild increase in low-mass or stellar remnants is therefore also consistent with this M/LV . The stellar density of the galaxy is so high that no dynamical signature of is expected. However, the properties of M60-UCD1 suggest an origin in the tidal stripping of a nucleated galaxy with MB ∼−18 to −19. Key words: : dwarf – galaxies: elliptical and lenticular, cD – galaxies: individual (M60) – galaxies: kinematics and dynamics – galaxies: clusters: general Online-only material: color figures

1. INTRODUCTION There is more at stake than the natural desire to understand these novel stellar systems. If a significant fraction of UCDs Objects with sizes and masses between those of globular contain dark matter, then they form a populous class that clusters and compact ellipticals (rh ∼ 10–100 pc; MV ∼−9to must be included in counts of subhalos for comparisons to −14) were first discovered in spectroscopic surveys of galaxy cosmological theory. Further, if some UCDs are formed by tidal clusters (Hilker et al. 1999; Drinkwater et al. 2000). They were stripping, their chemical and structural properties help trace quickly dubbed “ultra-compact dwarf ” galaxies (UCDs), even galaxy transformation. though their galaxian was unclear. Large populations of Here we report the discovery of an extraordinary UCD around UCDs have been discovered in , Virgo, and other galaxy the Virgo elliptical NGC 4649 (M60). It has a half-light radius 8 clusters, as well as in group and field environments—see reviews of 24 pc but a of 2 × 10 M0,givingitthe in Chilingarian et al. (2011), Norris & Kannappan (2011), and highest surface density of any galaxy in the local universe. Brodie et al. (2011). We also present evidence that this UCD may contain a central UCD formation scenarios have coalesced around two poles: . or galaxy. In the former scenario, UCDs form the massive end of the normal sequence of globular clusters (Mieske 2. DATA et al. 2012). Further, if some star clusters form in gravitationally 2.1. Imaging bound complexes, these can merge to make objects that are larger and more massive than single clusters (Bruns ¨ et al. 2011). We discovered M60-UCD1 in the Hubble Alternatively, UCDs could be galaxies that formed in individ­ (HST)/Advanced Camera for Surveys (ACS) imaging of Strader ual dark matter halos—either “in situ,” as unusual, extremely et al. (2012). We have a single orbit of imaging split between compact galaxies—or as the products of tidal stripping of more F 475W and F 850LP (hereafter g and z). M60-UCD1 is massive progenitor galaxies (e.g., Drinkwater et al. 2003). located at (R.A., decl.) = (190.8999, 11.5347) in decimal J2000 A reasonable synthesis of these scenarios may be that the coordinates. This is at a projected distance of only ∼6.6 kpc from 6 least-massive “UCDs,” with ∼10 M0, are largely star clusters, the center of M60 (Figure 1; assuming a distance of 16.5 Mpc; 8 while the most massive objects (210 M0) are galaxies, or the Blakeslee et al. 2009). No mention is made of M60-UCD1 in tidally stripped remnants thereof. At intermediate masses both previous Virgo surveys, including the ACS Survey star clusters and galaxies may coexist (e.g., Norris & Kannappan (Cot ˆ eetal.´ 2004). It is present in the 2011; Brodie et al. 2011). Data Release 7 photometric catalog (Abazajian et al. 2009)

1 The Astrophysical Journal Letters, 775:L6 (6pp), 2013 September 20 Strader et al.

Figure 1. HST/ACS color image of the central region of M60, showing the location of M60-UCD1 (solid circle). A typical UCD (A32, ∼3 × 106 M0; Strader et al. 2012) is also marked (dashed circle) for reference. (A color version of this figure is available in the online journal.) as J124335.96+113204.6, and was classified by Simard et al. empirical point-spread function (PSF) using a custom software (2011) as a background galaxy. package as described in Seth et al. (2006). The PSF (10× subsampled) was derived from point sources in the images. The 2.2. Spectroscopy fits were performed on a 511×511 image centered on the UCD, A spectrum of M60-UCD1 was obtained on the night of 2012 with the background galaxy gradient from M60 itself removed. January 17 with Keck/DEIMOS (Faber et al. 2003), utilizing the The fits are not very sensitive to the fitting box size. 1200 l/mm grating centered at 7800 Å and a 111 slit (resolution From the residual map (Figure 2), it is clear that a single ∼1.5 Å). We obtained three 30 minute exposures in 011.8 seeing. Sersic´ component provides a poor fit. In particular, the ellipticity Using the spec2d pipeline (Cooper et al. 2012), the spectra were of M60-UCD1 becomes more circular at larger radii, leaving a extracted, calibrated, and combined in the standard manner to residual along the minor axis. The radial shape of the surface produce a final one-dimensional spectrum. brightness profile is also poorly fit. To improve stellar population constraints, further spec­ However, using a two-component Sersic´ model, a very good 2 = troscopy was undertaken with MMT/Hectospec (Fabricant et al. fit (χν 1.07 in g) is obtained. Table 1 gives the parameters for 2005) on 2012 May 16, using the 270 l/mm grating with wave­ the single and double Sersic´ g-band fits and the double Sersic´ z length coverage from 3700 to 9100 Å and 5 Å resolution. Three fits. For the two-component fits the S ersic´ parameters n and re 20 minute exposures were taken in 011.9 seeing. These Hectospec are very similar between the filters. Because the g band provides data were pipeline-reduced in a standard manner as described a much better fit (probably due to PSF modeling issues in z), all in Mink et al. (2007). structural values cited are from the g fits. For the best-fit two-component model, the inner component = = 3. ANALYSIS AND RESULTS is compact (re 14 pc), with modest ellipticity (E 0.25), and has about 58% of the total of M60-UCD1. The outer 3.1. Imaging component is more extended (re = 49 pc), round, and with ∼ Aperture of M60-UCD1 gives a total integrated a nearly exponential profile (n 1.2). The overall half-light = ± of z = 15.86 ± 0.02 and g = 17.40 ± 0.02, radius is re 24.2 0.5 pc, empirically measured using the yielding g − z = 1.54 ± 0.03 (the g and z magnitudes in deconvolved g profile. This value is similar to the radius derived from the single-component S ersic´ fit; within this Letter are AB). The measured half-light radius (see below) 7 this radius we estimate LV ∼ 2.1 × 10 L0. is re = 24.2 ± 0.5 pc. The inferred total are 7 7 Lg = (3.26±0.06)×10 L0; Lz = (7.88±0.14)×10 L0; and 7 3.2. Spectroscopy LV = (4.12 ± 0.08) × 10 L0. With MV =−14.2, M60-UCD1 is the most luminous UCD known (see Section 4.1 for further 3.2.1. Dynamical Mass, Mass-to-light Ratio, and Resolved Kinematics discussion). We fit the optical imaging with one- and two-component Using the Keck/DEIMOS spectrum, the integrated velocity elliptical S ersic´ models. These fits, shown in Figure 2,were dispersion of M60-UCD1 was measured by cross-correlating performed by fitting two-dimensional models convolved with an the region around the Ca triplet with a library of templates of

2 The Astrophysical Journal Letters, 775:L6 (6pp), 2013 September 20 Strader et al.

Table 1 Surface Brightness Profile Fits

2 a Component χ /ν Total Luminosity μe re n b/a P.A. − ◦ (AB mag) (L0) (mag arcsec 2)(pc) ( ) g Single 2.33 17.51 2.92 × 107 18.32 ± 0.01 27.3 ± 0.1 3.53 ± 0.01 0.870 ± 0.001 −49.2 ± 0.3 g Double (inner) 1.07b 18.11 1.69 × 107 17.35 ± 0.08 14.3 ± 0.7 3.32 ± 0.08 0.750 ± 0.004 −47.0 ± 0.3 g Double (outer) ... 18.46 1.22 × 107 20.13 ± 0.06 49.1 ± 0.5 1.18 ± 0.03 0.964 ± 0.005 −10 ± 9 z Double (inner) 2.03b 16.55 4.17 × 107 15.77 ± 0.06 14.6 ± 0.4 3.28 ± 0.06 0.708 ± 0.004 −49.4 ± 0.3 z Double (outer) ... 16.89 3.05 × 107 18.57 ± 0.04 50.4 ± 0.5 1.14 ± 0.02 0.930 ± 0.007 109 ± 3

Note. a Errors are dominated by the sky determination and are <0.05 mag. b χ 2/ν applies to both components.

of Sersic´ profiles, we assume β = 7, intermediate between the applicable values for the n = 3.3 and n = 1.2 profiles (corresponding to the inner and outer components, respectively). −1 We further estimate that σe = 71 ± 5kms , slightly higher than the measured value of σp, by integrating over our DEIMOS extraction window (111. 2 × 111.0) and accounting for seeing. The dynamical mass determined in this manner is 8 Mvir = (2.0 ± 0.3) × 10 M0. The systematic uncertain­ ties are significant: we have assumed isotropy, sphericity, and mass-follows-light. Dividing this dynamical mass by the total luminosity of M60-UCD1 yields a mass-to-light ratio of M/LV = 4.9 ± 0.7. The flexible stellar population synthesis models of Conroy et al. (2009), using Padova isochrones and a Kroupa initial mass function (IMF), predict M/LV = (3.5, 4.2, 4.7) for solar metallicity and ages of (8, 10, 12) Gyr, respectively.8 If M60-UCD1 has a younger age, the dynamical M/LV could imply an elevation in low-mass stars or stellar remnants over Kroupa IMF model predictions. For older ages there is an excellent match between the observed M/LV and the model predictions. As discussed in Section 3.3, a modest increase in the central velocity dispersion (and hence M/L) could also be caused by the presence of a supermassive black hole with a mass ∼10% of that of the UCD (Mieske et al. 2013). Dark matter is not expected to contribute to the M/L (Section 4.2). M60-UCD1 is marginally resolved in our DEIMOS observa­ tions, and so some spatially resolved kinematic information is available. The 511 slitlet was aligned close to the major axis of the object. Using the sky-subtracted two-dimensional spectrum, we determined the and velocity dispersion on a Figure 2. Top panel: the two-dimensional residuals of the best-fitting single pixel-by-pixel basis (one pixel is ∼011.12). There is clear rota- S ersic´ (left) and double S ersic´ (right) fits. Contours show the g surface brightness − −2 ∼ 1 at μg = 17–23 mag arcsec . Bottom panel: one-dimensional profile showing tion present, with an amplitude of 30 km s to a projected the results of our two-dimensional fits to the g surface brightness profile of radius of ∼111.1. A decline of comparable amplitude in the ve­ M60-UCD1. The fit is shown in the top panel and the residual (data minus locity dispersion is also observed. Since the radial profiles are model, units of mag arcsec−2) in the bottom panel. Open black diamonds show the data and residuals, green dashed and blue dot-dashed lines the inner and strongly affected by seeing, we do not attempt dynamical model- outer components, and solid red line the sum. All fits were performed in two ing. However, these data provide motivation to obtain improved dimensions; these profiles are for display only. kinematic maps in the future using integral-field spectroscopy. (A color version of this figure is available in the online journal.) 3.2.2. Abundances We constrain the stellar populations of M60-UCD1 using our the same resolution and wavelength coverage, as described by MMT/Hectospec spectrum (with its wide wavelength range) = ± −1 Strader et al. (2011). This value is σp 68 5kms .The and the models of Conroy & van Dokkum (2012a; with radial velocity of M60-UCD1 is 1290 ± 5kms−1; the systemic − additions from Conroy & van Dokkum 2012b). These are stellar velocity of M60 is 1117 km s 1(Gonzalez´ 1993). population synthesis models with variable abundance ratios for We estimate a dynamical mass for M60-UCD1 using the virial 11 elements. A Markov Chain is used to = 2 theorem: Mvir βσe re/G. β is a parameter that depends on simultaneously fit the entire available optical spectrum. the structure of the galaxy and is smaller for more concentrated 8 systems; σe is the integrated velocity dispersion within re. Using a Salpeter IMF gives M/LV = (5.9, 7.0, 7.9) for these ages, an Following the results of Cappellari et al. (2006) for a range increase of nearly 70% in stellar mass over a Kroupa IMF.

3 The Astrophysical Journal Letters, 775:L6 (6pp), 2013 September 20 Strader et al. Table 2 over timescales as short as a few months. The X-ray spectrum M60-UCD1 Abundances is well fit by a absorbed power law with a photon index of 1.8. Element Full Spec. Lick There are two reasonable possibilities for this central X-ray (dex) (dex) source: it could either be an active galactic nucleus associated [Fe/H] −0.02 ± 0.02 +0.06 ± 0.03 with a massive black hole or a low-mass X-ray binary. [O/Fe] +0.19 ± 0.07 ... The case for a central black hole is straightforward. If the [C/Fe] +0.10 ± 0.03 +0.02 ± 0.04 black hole occupation fraction of dwarf galaxies is high, and if [N/Fe] +0.61 ± 0.04 ... UCDs are the products of tidal stripping of dwarf galaxies, then [Na/Fe] +0.42 ± 0.03a ... a significant fraction of UCDs should have “overmassive” black [Mg/Fe] +0.22 ± 0.02 +0.26 ± 0.03 holes that could be detected through dynamical or [Si/Fe] +0.12 ± 0.05 ... signatures. If UCDs have been stripped of 99% or more of their [Ca/Fe] +0.03 ± 0.02 −0.01 ± 0.02 original mass (we estimate in Section 4.2 that the progenitor of ± [Ti/Fe] +0.16 0.03 ... M60-UCD1 was ∼50–200 times more massive), then they could 2 a host supermassive black holes that are 10% of their present- Note. This abundance is largely determined by the resonance day masses (Mieske et al. 2013). Frank et al. (2011) constrain a doublet at 589 nm, but the 819 nm Na i line gives a consistent result. putative black hole to be <5% of the total mass of one Fornax cluster UCD through integral-field spectroscopy. The derived values are listed in Table 2. The uncertainties The observed X-ray luminosity would be consistent with a 7 −4 quoted are solely statistical, and do not include the substantial ∼10 M0 black hole accreting at 10 of the Eddington rate systematic uncertainties necessarily present in any integrated- with a radiative efficiency of 10−3 . This Eddington ratio of light study of stellar populations. The rms residuals in the 10−7 would be typical of nuclei with old stellar populations (Ho fit were <1% over most of the spectrum. The formal age is 2009). 14.5 ± 0.5 Gyr, indicating an old stellar population. We can also estimate the odds that M60-UCD1 contains a M60-UCD1 is of solar metallicity with a mild elevation in bright X-ray binary. Sivakoff et al. (2007) derive formulae to [α/Fe] over solar. The abundances for C, O, and α-elements estimate the probability that a contains a low- 38 −1 appear very similar to the mean values for high-σ local early- mass X-ray binary with LX > 3.2 × 10 erg s . The odds type galaxies determined in a similar manner (Conroy et al. are higher for metal-rich clusters and those with high encounter 2013). However, the abundance of N is unusual: it is strongly rates. Applying their results, but extrapolating to the fainter lu­ enhanced, with [N/Fe] ∼ +0.6, comparable to the average value minosity observed, suggests a ∼25% chance of having observed observed in globular clusters (e.g., Briley et al. 2004). The high a low-mass X-ray binary in M60-UCD1. However, these results abundance of N in globular clusters is generally attributed to are of uncertain relevance for an object with a different struc­ self-enrichment by the winds of stars ture and formation history than a globular cluster (since what is (Gratton et al. 2012). Our results suggest light-element self- pertinent is the integrated—not instantaneous—collision rate). enrichment may also be present in UCDs, presumably related Dabringhausen et al. (2012) suggest UCDs have a lower occur­ to their high stellar densities. rence of low-mass X-ray binaries than expected on the basis of We note that [Na/Fe] varies strongly with σ in early-type the Sivakoff et al. (2007) results. galaxies, increasing from ∼0to ∼+0.4forσ ∼ 140 to 300. The Future observations can help clarify the nature of the X-ray 6 M60-UCD1 abundance ([Na/Fe] ∼ +0.4) is therefore difficult source. For example, if M60-UCD1 hosts a 210 M0 black hole to interpret: it could be expected, or could represent a large that lies on the radio–X-ray fundamental plane for black holes enhancement over baseline. (Plotkin et al. 2012), it should be detectable with the Very Large As a check on these values, we performed a standard Lick Array. index analysis using EZ_Ages (Graves & Schiavon 2008). These values are also listed in Table 2, and the same caveats apply. 4. DISCUSSION This analysis gave a formal age of ∼9–11 Gyr and similar abundance values for most of the elements in common with 4.1. The Densest Galaxy? the full-spectrum analysis (C, Ca, Mg, Fe). For N the EZ_Ages Σ analysis does not yield a reliable value, as the CN index strength Figure 3 shows a plot of log versus log LV for dispersion- supported stellar systems. Σ is the mean surface luminosity is off the grids. The Lick CN2 index for M60-UCD1 (0.24 mag) is comparable to that in most metal-rich M31 globular clusters, density within re. Globular clusters are plotted with different which are also thought to be self-enriched (Schiavon et al. 2012). symbols than galaxies. It is clear that M60-UCD1 is an unusual The very high abundance of N appears to be a robust object: it is much denser than any other object classified as conclusion of the analysis. a galaxy. It is more massive than any UCD or star cluster of comparable size, but is much more compact than other galaxies 3.3. X-ray Data: Central Black Hole or X-ray Binary? of similar luminosity. M60-UCD1 is not the densest stellar known. That An X-ray source at the position of M60-UCD1 is present in honor goes to any of a number of nuclear star clusters, which 5 −2 the Chandra/ACIS catalog of Luo et al. (2013). The central can reach mean surface densities of >10 M0 pc within re astrometric matching between the Chandra and HST data is (Walcher et al. 2005; these are not plotted in Figure 3). Many excellent due to the large X-ray binary and globular cluster massive globular clusters are also extremely dense. However, populations of M60, with an rms scatter of 011.17. M60-UCD1 is arguably the densest galaxy known in the local This X-ray source, called XID 144 by Luo et al. (2013), has a universe. Using the M/LV from Section 3.2.1, its mean effective 4 −2 position consistent with the optical center of M60-UCD1. There surface density is Σ = 5.4 × 10 M0 pc , a factor of 2.5–3 is evidence that it is variable, with its X-ray luminosity (from higher than for M32. The inner component of M60-UCD1, with 37 38 −1 4 −2 0.3 to 8 keV) ranging from ∼6 × 10 to ∼1.3 × 10 erg s re ∼ 14 pc, has a mean Σ ∼ 9 × 10 M0 pc , comparable to

4 The Astrophysical Journal Letters, 775:L6 (6pp), 2013 September 20 Strader et al.

The inner component of M60-UCD1 has Mg =−13.1, g − z = 1.54, and re = 14 pc. The size, luminosity, and red color are similar to nuclei in the Virgo galaxies NGC 4379 and NGC 4387 (Cot ˆeetal. ´ 2006) and the galaxies NGC 1389 and IC 2006 (Turner et al. 2012). These galaxies 10 have MB ∼−18 to −19 and stellar masses ∼1–3 × 10 M0. We conclude that M60-UCD1 could have originated in the tidal stripping of a Virgo galaxy in this luminosity range. Since the present luminosity of M60-UCD1 is MB ∼−13.2, we infer that it is a factor of ∼50–200 less massive because of the stripping. The projected distance of M60-UCD1 from the center of M60 (∼6.6 kpc) is consistent with the small pericenter needed for efficient stripping (Pfeffer & Baumgardt 2013). Galaxies in this mass range host significant globular cluster populations (∼40–100; Brodie & Strader 2006) that would also be stripped during UCD formation and might still be detectable in phase space (e.g., Romanowsky et al. 2012). We caution that UCDs may originate in a biased subset of host galaxies that have been largely destroyed, so it is possible that there is no correspondence between UCD progenitors and a subpopulation of surviving galaxies. The Pfeffer & Baumgardt (2013) simulations show that ex­ tended debris suggestive of tidal stripping becomes challenging to observe after relatively short (∼1 Gyr) timescales, so the lack Figure 3. log Σ (mean surface luminosity density within re)vs.log LV for dispersion-supported stellar systems (GC = globular cluster; cE = compact el­ of evidence for such debris in Figure 2 does not disfavor this liptical; E = early-type galaxy; dE = dwarf elliptical). The inner component and scenario. The dynamical friction timescale (Binney & Tremaine overall parameters for M60-UCD1 (red stars) are marked, as are the comparison 1987) for M60-UCD1 is ∼5 Gyr, so it is plausible that its pro­ objects M32 and the luminous M31 cluster G1 (blue squares). Globular clusters genitor was stripped long ago and the remnant has “stalled” at − (the union of objects with re < 10 pc and non-dwarf galaxies with MV > 9; its current radius. In this case no observable tidal tails would be Brodie et al. 2011) are small points; galaxies are large points. M60-UCD1 has a higher Σ than any other galaxy. The black arrow represents the proposed evo­ expected. lution of the progenitor of M60-UCD1 as it was tidally stripped. The principal The stellar density of M60-UCD1 is much higher than data source for this figure is the spectroscopically confirmed compilation of expected for dark matter with a standard Navarro–Frenk–White Brodie et al. (2011;see http://sages.ucolick.org/downloads/sizetable.txt), with profile (a factor of ∼15 for even for extreme, cluster-scale halo updates from Forbes et al. (2013). masses; Tollerud et al. 2011). If the galaxy has undergone as (A color version of this figure is available in the online journal.) much tidal stripping as inferred, this is likely to have strongly modified the dark matter profile; nonetheless, M60-UCD1 is that of many nuclear star clusters. The central volume density probably the worst UCD in which to search for dark matter. As of M60-UCD1 is not well constrained by the present data. discussed by Willman & Strader (2012; see also Hilker et al. The object most similar to M60-UCD1 is HUCD1, a 2007), the UCDs most likely to show evidence for dark matter are the least massive and most extended UCDs. Cluster UCD, which has re = 25 pc and MV =−13.4 (Misgeld et al. 2011), though M59cO (Chilingarian & Mamon 2008) and Future observations will help constrain the detailed properties several UCDs (Chiboucas et al. 2011)arealso of M60-UCD1, including its two-dimensional kinematics and similar, if less extreme. It seems likely that ongoing surveys for whether it hosts a central supermassive black hole. UCDs will turn up additional objects with properties comparable to M60-UCD1. We thank L. Chomiuk, S. Mieske, and R. Schiavon for useful discussions and an anonymous referee for a helpful report. Based on observations made with the NASA/ESA Hubble Space 4.2. The Origin of M60-UCD1 Telescope and the Hubble Legacy Archive. Data obtained at The extreme mass, multiple structural components, high Keck (Caltech/UC/NASA) and MMT (Arizona/Smithsonian). metallicity, and possible presence of a central black hole make Products produced by the OIR Telescope Data Center (SAO). it unlikely that M60-UCD1 is a star cluster or merged cluster Support by ARC grant DP130100388 and NSF grants complex. It is most plausible that the object is the tidally stripped AST-1109878/AST-0909237. remnant of a more massive progenitor galaxy. 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